Posted on Feb 7, 2014

The photometric camera for the SDSS consists of two TDI scanning CCD arrays, one, using 30 Tektronix/SITe 2048 x 2048 CCDs in a 5 by 6 array for five-color photometry, and the other using 24 2048 x 400 chips for astrometry and focus monitoring. We here discuss these arrays, the filter system, the scheme for focusing the telescope during an imaging

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scan, and the CCDs and associated electronics for the camera and the spectrograph. There are several important considerations in selecting the filter system. First, one wants the largest color baseline possible. It is desirable but not necessary to use filters for which there already exist a large amount of data. For example, the Thuan-Gunn (1976), (see also Schneider et al. 1983) uvgriz system, which has been used for much of the work at Palomar on distant clusters, avoids the 5577G… night sky line, and the i band cuts off in the near-infrared just shortward of the major OH bands. The g and r filters more-or-less match the J and F bands of the new Palomar Sky Survey. The uvgriz filters provide coverage of the total wavelength interval and are almost uniformly spaced in log lambda. They are, however, fairly narrow, and all except u could be usefully widened for a system like ours in which efficiency is of paramount importance. We have designed a filter system similar to ugriz which should transform to and from it with little difficulty, in which the g, r and i filters are as wide as practicable consistent with keeping the overlap small. If we wish the u band to be a "good" one which is almost entirely contained between the Balmer jump and the atmospheric cutoff (we can clearly not do anything about that end), it cannot be significantly wider than Thuan-Gunn u. We call the new system u` g` r` i` z`. The response curves and...

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